1. Field of the Invention
This invention relates to welding guns and contact tips for welding guns and, in particular, to a MIG welding gun with a contact tip.
2. Description of the Related Art
A metal inert gas (MIG) welding gun generally comprises a handle, a wire feed mechanism, and a torch barrel. A push-pull MIG welding gun can also include a motor. The welding gun typically also includes a gas tube and a cooling fluid tube for supplying gas and cooling fluid to the tip of the torch barrel. There are three general types of welding guns: pistol grip, goose neck and in-line welding guns. In a pistol grip gun, a motor can be located below a wire feed mechanism and can lie along an axis that lies generally perpendicular to the longitudinal axis of the gun. In contrast, in an in-line gun, a motor can lie along the longitudinal axis of the gun and is positioned generally behind the wire feed mechanism.
In MIG welding, the welding gun typically has a contact tip that preferably performs two functions. First, it accurately guides a weld wire to the point of contact with the workpiece. Second, it conducts adequate current, which may be as high as 400 amps or more, to the weld wire. A MIG gun has an electrical hot wire at the tip. To achieve the two functions of a MIG gun, a relatively long contact tip made of copper can be provided.
The contact tip can have an axially extending hole or bore provided therethrough. The hole preferably is sized slightly larger than the diameter of the weld wire. The contact tip can be coupled to and extend from a gas diffuser. An inert gas can be directed around the welding arc through the gas diffuser. Current can be supplied to the contact tip from the power block through the barrel or the gas diffuser.
Contact tips are particularly subject to at least two modes of failure. The first is burnback, which occurs if the feeding of weld wire through the contact tip is stopped while an arc is maintained, thus melting the wire back towards the end of the tip and gun. The loss of wire contact, wire diameter, alloy type, thermal expansion of wire and energy input may affect burnback. The second mode of failure is wearing of the through hole due to the abrasive nature of the weld wire material. In addition, because the contact tip is so close to the welding arc, and because the tip may carry high electrical currents, it may become very hot. The high temperatures also have a deleterious effect on the contact tip, due in part because the hotter the tip becomes the softer it becomes, which makes it less resistant to wear from the weld wire.
Due to the severe operating conditions, contact tips have relatively short service lives. Consequently, they are treated as consumable items in the industry. Typically, a contact tip is made as a separate piece that can be threaded into a gas diffuser. When a contact tip has worn, it is removed from the gas diffuser and discarded. A new contact tip is then coupled to the welding gun.
Removing a worn contact tip from the gas diffuser can require several turns of the contact tip. If the weld wire has burned back and fused to the contact tip, the weld wire is typically required to twist through the same number of turns as the contact tip. The potential springback of the fully removed contact tip due to the twisted weld wire is annoying if not potentially dangerous. Moreover, the time required to disassemble the contact tip also contributes to downtime of the welding production area. Newer pulsed and adaptive computer programs can monitor the current flowing through the contact tip to provide feedback or rapid (e.g., instantaneous) adjustment regarding the consistency of the contact. Providing a contact tip that has consistent contact can reduce failures and make the entire system more cost effective.
As stated above, a contact tip preferably conducts adequate current to the weld wire. In order for current to flow from the contact tip to the weld wire, the weld wire preferably remains in contact with the contact tip. However, as described above, the hole in the contact tip preferably is sized slightly larger than the diameter of the weld wire. Accordingly, in some cases the size of the hole may start out too large and contact between the weld contact tip and the weld wire is not continuously maintained. Depending on the size of the weld wire and the size of the hole, the weld wire generally comes in and out of contact with the contact tip during the welding process at totally random intervals and durations. When contact with the contact tip is lost, the contact tip ceases to transfer current to the wire. Even a momentary loss of current through the weld wire is undesirable. Where current has not been adequately maintained, leading to irregularities in the weld, workers may be required to cut out a section of a poor weld or x-ray the weld to ensure adequacy. This can require a large number of hours and can become very expensive.
Reducing the size of the hole to minimize the loss of contact can also provide significant problems. For example, the size or dimensions of the weld wire may vary somewhat due to manufacturing tolerances or thermal expansion. When the hole is too small, the weld wire can become lodged in the contact tip. If the weld wire becomes lodged in the contact tip it may fuse with the contact tip. As stated above, burnback can cause the contact tip to fail if the feeding of weld wire through the contact tip is stopped while an arc is maintained.
Some contact tips provide a radial opening for receiving a clamping ball and a tubular stainless steel split clip to hold the clamping ball against the weld wire to maintain contact between the weld wire and a side of the contact tip hole.